1. Field
The present disclosure relates to techniques for modulating optical signals. More specifically, the present disclosure relates to an integrated optical device that includes a low-voltage ring resonator that includes a ferroelectric layer.
2. Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip optical interconnects or links. Key components for use in inter-chip and intra-chip optical interconnects are modulators, filters and wavelength-division-multiplexing (WDM) components that can be integrated in the same silicon layer as other optical components and transistors.
Silicon-photonic ring resonators can operate at infrared wavelengths that include a 1.5 μm band, which makes them suitable for use as modulators and filters. In multi-wavelength applications, such ring resonators are modulators but they also serve as multiplexers and de-multiplexers to enable signaling on different wavelength channels for dense WDM links. Moreover, the silicon ring resonators can be fabricated using CMOS-compatible silicon-on-insulator (SOI) technology that confines the optical mode in a compact footprint so that bending losses are low enough to support a high density of interconnects on a chip. These high-density ring resonators can be fabricated in typical, narrow linewidth nodes at CMOS foundries, thereby offering the advantages of high volume and low cost.
However, as with transistors, silicon ring resonators are subject to manufacturing variations. While the impact of manufacturing variations on electrical components is typically within the noise margins, for optical applications these manufacturing variations can pose a more serious challenge. In particular, manufacturing variations in the fabrication of ring resonators include variations in: the etch depth, the etch width and the thickness of the silicon layer above the buried-oxide layer that geometrically defines a ring resonator. These variations result in different effective indexes of refraction for ring resonators, which in turn changes the group velocity of the light that transverses the ring resonators. The main impact of such changes is to shift the resonant wavelength of the ring resonators away from their intended target values in an unpredictable way. In some cases the error can be large enough so that the resonant wavelength extends all the way to the next higher-order resonance of a ring resonator, or more than one free spectral range (FSR) away from the target value.
This unpredictable shift in the resonant wavelength of the ring resonators can make WDM applications difficult because the resonant wavelengths are expected to fall within and on a uniform grid of wavelengths (which is sometimes referred to as an ‘International Telecommunication Union grid’ or ‘ITU grid’). Typically, ITU grids vary between 0.8 and 10 nm in wavelength spacing for dense WDM to coarse WDM applications. Thus, the manufacturing variations in ring resonators can make it difficult to align resonant wavelengths in different components (such as multiplexer and de-multiplexer ring-resonator filters) and/or with the carrier wavelengths output by a set of optical sources.
In principle, ring resonators can be tuned so that their resonant wavelengths match their target values. However, in practice, such tuning can significantly increase power consumption. Indeed, the tuning power for ring resonators can be larger than those of any other power component in a silicon-photonic link (including the power used to modulate and detect the light), and can even exceed optical losses associated with propagation of the light in silicon optical waveguides. Moreover, the increased power consumption can result in increased temperatures in chips with high interconnect density, and thus may present challenges for existing thermal-management techniques.
Hence, what is needed is an integrated ring resonator without the above-described problems.